1. Field of the Invention
The present invention relates to the reduction of electromagnetic noise generated by the magnetic vibration of the claw-shaped magnetic pole portion of a vehicle alternator having a Lundell-type rotor.
2. Description of the Related Art
FIG. 10 is a view showing the arrangement of a conventional vehicle alternator and FIG. 11 is an enlarged sectional view showing the vicinity of the poles and the bobbin of a rotor.
In the respective drawings, a rotor 1 is composed of a pair of claw-shaped poles 2 each having a plurality of claws 2a formed thereto, a bobbin 3 around which a coil 4 is wound, a rotary shaft 5 and a slip ring 6. The pair of poles 2 are disposed in confrontation with each other so that the claws 2a are meshed with other and the rotary shaft 5 is fixed by being press fitted into the axial center position of the claws 2a. The inner peripheries 2b of the base portions of the claws 2a are abutted against the outer periphery 3a of the bobbin 3 which is fixed between the pair of poles 2 by the press fitting force of the rotary shaft 5. A stator 7 is disposed to the outer peripheries of the poles 2 with a slight interval defined therebetween. A voltage controller is mounted to a brush holder 8. A pulley 10 is mounted to the rotary shaft 5 integrally therewith and fans 11 are mounted to the end surfaces of the poles 2 integrally therewith. A front bracket 12 and a rear bracket 13 are disposed to surround the rotor 1 while journaling the rotary shaft 5.
Next, operation of the conventional vehicle alternator will be described.
A current is supplied to the coil 4 through the brush holder 8 and the slip ring 6 and at the same time the pulley 10 is driven in rotation by a belt to thereby permit the stator 7 to generate an alternating current. The alternating current is converted into a direct current by a rectifier 9. While the vehicle alternator is in operation, a magnetic attracting force is imposed on the poles 2 by the magnetic flux in the interval between the poles 2 and the stator 7. The magnetic flux continuously changes as a time elapses and the magnetic attracting force acts on the poles 2 as a swinging force so that the poles 2 are swung as shown by the arrow in FIG. 11. The swing motion of the poles 2 generates electromagnetic noise.
One conventional approach to reduce the electromagnetic noise, employs a ring 14 abutted against the inside of the claws 2a to increase the rigidity of the poles 2, shifting the vibration to a high frequency region. However, when the frequency of a forced power extends over a wide range of frequency as in the case of vehicles, this countermeasure only shifts the timing at which a noise is generated by the vibration to the high frequency region side and does not become a basic solution for reducing the electromagnetic noise. Further since the number of parts is increased, the cost is similarly increased.
An object of the present invention is to solve the above problem by providing a vehicle alternator capable of effectively reducing electromagnetic noise.
In order to achieve this object, according to one aspect of the present invention, there is provided a vehicle alternator having a bobbin around which a coil is wound and which is mounted in Lundell-type poles, the vehicle alternator comprising a auxiliary vibrating body mounted to the poles so as to constitute a vibration system having two-degrees of freedom which is composed of the poles and the auxiliary vibrating body, wherein the auxiliary vibrating body has a natural frequency approximately equal to that of the poles.